In an electrical communication system, it is sometimes advantageous to transmit information (video, audio, data) in the form of differential signals over a pair of wires rather than a single wire, where the transmitted signal comprises the voltage difference between the wires without regard to the absolute voltages present. Each wire in a wire-pair is capable of picking up electrical noise from outside sources, e.g., neighboring data lines. Differential signals may be advantageous to use due to the fact that the signals are less susceptible to these outside sources.
When using differential signals, it is well known that it is desirable to avoid the generation of common mode signals. Common mode signals are related to a balance of the transmission line. Balance is a measure of impedance symmetry in a wire pair between individual conductors of the wire and ground. When the impedance to ground for one conductor is different than the impedance to ground for the other conductor, then differential mode signals are undesirably converted to common mode signals.
Another concern with differential signals is electrical noise that is caused by neighboring differential wire pairs, where the individual conductors on each wire pair couple (inductively or capacitively) in an unequal manner that results in added noise to the neighboring wire pair. This is referred to as crosstalk. Crosstalk can occur on a near end (NEXT) and a far end (FEXT) of a transmission line. It can also occur internally between differential wire pairs in a channel (referred to as internal NEXT and internal FEXT) or can couple to differential wire pairs in a neighboring channel (referred to as alien NEXT and alien FEXT). Generally speaking, so long as the same noise signal is added to each wire in the wire-pair, then the voltage difference between the wires will remain about the same and crosstalk is minimized.
In the communications industry, as data transmission rates have steadily increased, crosstalk due to undesired capacitive and inductive couplings among closely spaced parallel conductors within the jack and/or plug has become increasingly problematic. Modular connectors with improved crosstalk performance have been designed to meet the increasingly demanding standards. For example, recent connectors have introduced predetermined amounts of crosstalk compensation to cancel offending NEXT. Two or more stages of compensation are used to account for phase shifts from propagation delay resulting from a distance between a compensation zone and the plug/jack interface, which, in turn gives the system an increased bandwidth. Additionally, new standards have been particularly demanding in the area of alien crosstalk. Common mode signals are known to radiate more than differential signals, and therefore are a major source of alien crosstalk. Therefore, minimizing any sort of common mode signal is desirable, and this has driven the need for new connector designs.
Recent transmission rates, including those requiring a bandwidth in excess of 250 MHz, have exceeded the capabilities of the prior techniques for both internal NEXT and alien NEXT. Thus, improved compensation techniques are needed.